438 



NATURE 



[December 2, 1920 



a contributing factor in the further development and 

 maintenance of storm energy. 



To take the very fact which Mr. Dines cites, 

 namely, the exceptional storminess of the Atlantic to 

 the north-west of Scotland. This region is, in a 

 most conspicuous degree, stormier in the winter 

 months than in the summer, and it is almost one 

 of the canons of physical geography that the exces- 

 sive development of storm-energy during the cold 

 season is favoured by the great contrast in tempera- 

 ture between the frostbound continents and the warm 

 Atlantic, the individual cyclonic systems breeding not 

 so actively over the land areas, where the general 

 pressure is high, as over the oceanic areas, where 

 the general pressure is low. On the other hand, 

 during the warm season — when the temperature 

 gradient bet\?een the oceans and the continents is 

 reversed, but is much less steep than the winter 

 gradient — cyclonic energy in the North Atlantic is far 

 less powerful, whilst over the sun-heated continents 

 storm-energy takes the form, not of extensive wind- 

 systems, but of localised convectional thunder-systems. 

 Furthermore, in the southern ocean, between 40° and 

 60° S., where there are no disturbing land masses, 

 there does not appear, judging from the reports of 

 navigators, to be such conspicuous seasonal difference 

 in storminess, and this is borne out by statistics avail- 

 able for the Falkland Islands (Meteor. Oflfice Geophys. 

 Mem., No. 15). L. C. W. Bonacina. 



November 19. 



Sir Oliver Lodge's suggestion and mine in 

 Nature of November 25 are not contradictory, but 

 rather complementary. Work done bv the alternate 

 evaporation and condensation of moisture implies a 

 thermodynamic cycle. Both air and aqueous vapour 

 must, I think, play the part of working substance. 



J. R. Cotter. 



Trinity College, Dublin, November 26. 



Luminosity by Attrition. 



.^LLOW me to add to the list of minerals showing 

 this phenomenon one which I have already given in 

 my book "Diversions of a Naturalist." It is that 

 of corundum. I found that water-worn pebbles of 

 corundum (so identified in the department of 

 minerals of the Natural History Museum) gave flashes 

 of light when rubbed together, but required for this 

 result a heavier pressure than do pebbles of silica. 

 The same odour as that observed when silica is used 

 was produced. 



I may also repeat here what I have stated in my 

 book, that a spectroscopic examination of the 

 luminous flashes of quartz pebbles gave a continuous 

 spectrum and no detached bright lines. 



E. Ray I.ankester. 



November 28. 



•Some ton years ago when grinding down a thin slice 

 of limestone under water I was surprised to find that 

 the operation was accompanied bv faint flashes of 

 light which seemed to issue from certain spots of 

 superior hardness ; on examining the slice under the 

 microscope it was found that these spots consisted 

 of quartz. This led me to devise an apparatus by 

 which the luminescence could be continuouslv pro- 

 duced and so rendered a subject for precise observa- 

 tion. The substance to be examined was attached to 

 the free end of a hinged bar and adjusted so that it 

 rested against th" edfje of an emery, or. still better, 

 a carborundum, wheel which was rotated bv an elec- 

 tric motor. Of some forty minerals experimented 

 upon no fewer than eighteen emitted light while 

 NO. 2666, VOL. 106] 



being ground. Those that did not included all the 

 sulphides which were examined, viz. zinc blende, 

 cinnabar, antimonite, galena, copper pyrites, and 

 arsenical pyrites. Iron pyrites, of course, yielded 

 sparks, but these were not accompanied by tribo- 

 luminescence. Almost all the silicates emitted light, 

 e.g. orthoclase, labradorite, idocrase, garnet, 

 tourmaline (one variety, another did not), epidote, 

 zircon, topaz, and glass; several oxides, e.g. 

 corundum, magnetitCj haematite, cassiterite, quartz, 

 and flint ; light was also obtained from wavellite, 

 apatite, celestine, and barytes. But the most re- 

 markable results were obtained from fluorspar ; all 

 the varieties of this mineral which were examined 

 gave light, but one in particular, distinguished by its 

 green colour, emitted blue light, not only in great 

 quantity, but also of such persistency that the whole 

 periphery of the wheel was alive with it. 



Curiously enough, no electrical phenomena were 

 observed in any case ; an electroscope, possibly not 

 a very sensitive one, gave no signs even when fully 

 exposed to the current of dust driven off during 

 grinding. 



The light emitted was in most cases white, but 

 often coloured reddish or yellowish, and in a few- 

 instances bluish. It would be quite possible to 

 examine (as I did) the light with a spectroscope, and 

 after some preliminary trials I planned apparatus for 

 photographing the spectrum. The outbreak of the 

 war, however, put a stop to my experiments, and I 

 have not vet had time to resume them. 



W. J. SOLLAS. 



Stellar " Magnitudes." 



May I ask whether it is not time to overhaul and 

 improve the conventional specification of stellar magni. 

 tudes? 



When first introduced, on the basis of ordinal 

 numbers, the plan was natural enough ; a third magni- 

 tude was naturally inferior to a first, and a group of 

 some twenty stars could be considered as of the first 

 magnitude. 



But when it was found possible to measure and 

 specify magnitudes with numerical accuracy — by 

 instrumental means not, I confess, fully known to 

 me — so that a Variable could be said to decrease from 

 2-14 to 2-56, the cardinal number specification looked 

 inverted. Moreover, magnitudes less than unity 

 became necessary for the brighter stars, and a suffi- 

 ciently bright star would presumably have the magni- 

 tude o ; a nova, for instance, might blaze up from 

 magnitude 12 to magnitude zero, or even become of 

 negative brightness at the height of its career. 

 Indeed, I gather that a more recent system, of what 

 are called "absolute magnitudes," really does involve 

 negative numbers. 



Would it not be well to reconsider the convention 

 and devise something more convenient? 



Oliver Lodge. 



Higher Forestry Education for the Empire. 



The question has been recently raised by the 

 Government of India as to the advisabilitv of either 

 training the probationers for the Indian Forest Ser- 

 vice entirelv in India or confining the training to one 

 centre in this country. The question has come to the 

 front owing to the changes to be introduced in the 

 administration of India, under which a larger pro- 

 portion of Indians will enter the Indian Forest Service 

 in the future, it being therefore considered desirable 

 to train the European and Indian probationers all 

 together. The professional forestry opinion of Indian 



